Orthogonal Frequency-Division Multiplexing (OFDM) is a proven access technique for efficient user and data multiplexing in the frequency domain. One example of a system employing OFDM is Long-Term Evolution (LTE). LTE is the next step in cellular Third-Generation (3G) systems, which represents basically an evolution of previous mobile communications standards such as Universal Mobile Telecommunication System (UMTS) and Global System for Mobile Communications (GSM). It is a Third Generation Partnership Project (3GPP) standard that provides throughputs up to 50 Mbps in uplink and up to 100 Mbps in downlink. It uses scalable bandwidth from 1.4 to 20 MHz in order to suit the needs of network operators that have different bandwidth allocations. LTE is also expected to improve spectral efficiency in networks, allowing carriers to provide more data and voice services over a given bandwidth.
One of the key features in OFDM is the ability to perform frequency-selective scheduling of users, as happens in LTE. In this scheme, estimation of the channel frequency response must be performed in both uplink and downlink directions (and reported to the base station in the downlink case), so that schedulers can perform optimum allocation of resources by choosing the appropriate parts of the spectrum for each user.
To facilitate this, the base station estimates the uplink channel frequency response and additionally receives downlink channel quality reports from the users, in which an estimation of the downlink channel frequency response is included. Based on these reports the scheduler allocates resources trying to avoid parts of the spectrum with poor frequency response on a per-user basis.
However, the resource allocation procedure in prior art techniques usually assigns an overall Modulation and Coding Scheme (MCS) to the whole transmission (independently for each code-word in case of employing multiple spatial streams), according to the overall perceived channel quality. This MCS determines the modulation and coding rate to be applied to the whole information packet, with greater redundancy and lower-order modulations when experiencing poorer channel responses (and vice versa). If the bandwidth reserved for a single user is much greater than the channel coherence bandwidth, then the channel will exhibit significant fluctuations in frequency along the scheduled resources. In that case the MCS will have to be fitted to the average channel conditions rather than to the detailed frequency response as reported by the mobile users.
With the use of ever higher frequency bands, the trend in future cellular systems is to extend the usable system bandwidth up to several hundreds of MHz, as foreseen for the Fifth Generation of mobile communications (5G). Such large bandwidths will translate into similarly large bandwidth allocations for the users, especially in small cells with good radio conditions.
An example of a simplified procedure for channel encoding and modulation used in prior art techniques is illustrated in FIG. 1. Essentially, the information passes through a Forward Error Correction (FEC) encoder where redundancy is added to the original data for protection against channel impairments. Depending on the FEC code, and in order to adapt the physical block lengths to the available block sizes accepted by the FEC encoder, the input data may optionally enter in the form of a number of smaller blocks, denoted as “codeblocks”. The FEC Encoder accepts each of these codeblocks as inputs and performs forward error correction to each of them with a fixed coding rate (usually ½ or ⅓, depending on the system). An optional Rate Matching function then accepts the encoded blocks and matches their sizes to the available physical resources according to the chosen MCS, thereby applying a common overall coding rate. The rate-matched codeblocks enter a Constellation Mapping function, where bits are transformed into complex symbols according to the modulation scheme given also by the MCS. These symbols are mapped to physical resources in a Physical Resources Mapping function, thus resulting in a number of modulated subcarriers that comprise the OFDM signal in the frequency domain.
It must be noted that the operation of transforming the input data into smaller codeblocks is not essential in prior art techniques, as the FEC encoding operation can be directly applied to the input block in some encoding schemes. This may happen e.g. when the sizes of the input data blocks are fixed in the system, or when the performance of the FEC encoder does not change significantly over the range of input sizes foreseen in the system. The same happens with the Rate Matching function, as it is only intended to adapt the variable input sizes to the available physical resources when there is a significant variability in any of them. The examples and figures in the proposed invention are only included for ease of explanation, but are not intended to restrict the applicability of the proposed ideas as will be explained in following sections.
Problems with existing solutions are that the demand for wider bandwidths aggravates the effect of frequency-selective radio channels, especially in outdoors where the coherence bandwidth is usually much smaller than the system bandwidth because of the large delay spread of the channel.
Traditional solutions to cope with small coherence bandwidths involve reserving only part of the spectrum for resources allocation in order to avoid nulls in the channel. In current LTE macro deployments this solution fits well with the user expectations, as users are unlikely to demand very large bit rates for high-quality services and thus are not allocated large portions of the spectrum. However when using large system bandwidths, and especially in small cells deployments, large bandwidth allocations can be expected for video services.
LTE base stations usually try to allocate the whole system bandwidth to the active users in the cell in order to avoid under-utilization of resources. This brings significant channel variations within the scheduled resources that should be tracked by the encoding and modulation scheme. The MCS will suit the average channel conditions along the allocated spectrum rather than the instantaneous frequency response. This effect reduces the effectiveness of the coding scheme and its resilience against multipath.
Apart from that, current resource mapping procedures in some technologies (like e.g. LTE) do not allow differentiated channel coding and modulation for each of the blocks comprising a given packet, because each block is spread along the scheduled bandwidth and thus cannot be given a separate MCS but rather an average one.
More specific solutions for efficient modulation and coding schemes are therefore needed when deploying large system bandwidths in OFDM under frequency-selective radio channels.